1. Field of Invention
This invention relates generally to optical information storage systems for the storage and retrieval of information and, more particularly, to an information storage medium for use in an optical storage system making use of magneto-optical materials for the storage of information. Magneto-optical materials can be used in optical storage disks in which new data can be written over locations on which data has already been stored.
2. Description of the Related Art
The optical information storage systems of the type generally referred to as magneto-optical storage systems are of particular interest because of the ability to alter or over-write information already stored on the medium. In these systems, a magnetic medium exhibiting the Kerr effect is used to store information. The Kerr effect provides that magnetic materials having a component of magnetic orientation parallel to a direction of impinging radiation will affect circularly polarized reflected radiation depending upon whether the magnetic orientation is parallel or anti-parallel to the direction of impinging radiation. With the proper optical components, the change in circularly polarized radiation can be used to provide a change in planar polarized radiation. Although the difference in the rotation of the plane of polarization is typically less than 2.degree., this difference has been found to be sufficient to provide a basis for an optical storage and retrieval system. In optical storage systems of this type, the orientation of a region of magnetic material in one direction represents a first logical state while the orientation of a region of magnetic material in the opposite direction represents a second logical state.
In an optical storage medium, one desirable characteristic is the ability to write directly on the storage medium without the requirement of preliminarily erasing the data already stored on the disk.
Various techniques and materials have been proposed for implementing the direct overwriting of magneto-optical disks. For example U.S. Pat. No. 4,882,718, U.S. Pat. No. 4,679,180, and European Patent 316,188, all issued to H. P. D. Shieh et al; Appl. Phys. Lett. 49, 473 (1986) by H. P. D. Shieh et al; J. Appl. Phys. Lett. 67, 447 (1990) by Takayama et al; J. Appl. Phys. 67, 4420 (1990) by Hashimoto et al. describes single layer direct overwrite techniques. At the present, the single layer direct overwrite technique provides a relatively low signal-to-noise ratio. In U.S. Pat. No. 4,855,975, Jpn. J. Applied Phys. 26, (1987), and JP. 86,239,652, all being disclosed by J. Saito et al., a storage technique having two laminated magneto-optical layers provides that the memory layer has a high room temperature coercivity and a low Curie temperature; while the reference layer has a low room temperature coercivity and a high Curie temperature. The storage medium requires two magnets, a low field bias magnet for writing/erasing, and a high field magnet for initializing the reference layer. By modulating the intensity of the laser radiation beam between low and high power levels, information can be recorded while simultaneously erasing prestored information. In Jap. J. Appl. Phys. 28, Suppl. 28-3, p. 367 (1989) by Iida et al., a two layer read write scheme is described. In the two layer storage media, the two magneto-optical layers are strongly exchange coupled and require a large initializing field, typically greater than 5.5 kOe. In "Optical Data Storage" J. Appl. Phys. 67, 4415 (1990) by K. Aratani et al., a trilayer storage structure has been proposed, at least in part to reduce the strength of the initializing field. However, even this structure requires an initializing magnet of more than 3 kOe.
More recently, a direct overwrite structure has been described in which the initializing field has been eliminated, cf. Jpn. J. Appl. Phys. 28-3, (1990) 371, by T. Fukami et al. In this structure, the reference layer has a much higher Curie temperature (&gt;400.degree. C.) than the memory layer Curie temperature (=190.degree. C.). The reference layer is initialized after fabrication (i.e., deposition) and the magnetization of this layer does not reverse, even during recording, because of the high Curie temperature. However, the overwriting performance is not disclosed. In J. Appl. Phys, 67 (1990) 4415 by T. Fukami, a four layer structure is disclosed which includes a memory layer, writing layer, switching layer, and initializing layer. The disclosed structure requires only a bias magnet for writing/overwriting.
In the article entitled Direct Overwrite Using Through Thickness Temperature Gradient, IEEE Transactions on Magnetics, Vol. 27, No. 6, November, 1991, T. Ohtsuk et al describe a direct overwrite storage structure using two magnetic layers. The reference layer retains a constant magnetic orientation during an operation of the structure. The implementation described in this reference also requires that the two layers each have a high magnetic anisotropy.
In the field of magneto-optical materials, a need has been felt for a ever increasing density for stored data. Recently, A. Fukumoto et al, SPIE Proceedings, Vol. 1499, page 216, (1991), and K. Aratani et al, SPIE Proceedings, Vol. 1499, page 209, (1991) have reported high resolution (sometimes referred to as super-resolution) media. The reported media has much higher bit densities than the conventional single layer magneto-optical disk. This disk requires an initializing magnet in addition to a regular write magnet. Finally this media does not have the direct over write capability.
In U.S. Pat. No. 4,649,519, entitled "Self Biasing Thermal Magneto-optical Medium", issued in the name of Sun et al., self-biasing magneto-optical storage structure is described. In the Sun reference, the interaction between the magnetically active layers is through a magnetic field and not by exchange coupling of the layers. In addition, the implementation disclosed by this reference requires the under layer to have a compensation point.
A need has been felt for a relatively simple magneto-optical structure or medium having the capability of directly overwriting already stored information and which does not require an initialization magnet for the operation.